Arc-image zone-refining furnace

ABSTRACT

An arc-image zone-refining furnace having an auxiliary spherical mirror located opposite to an ellipsoidal mirror. Provision of the auxiliary spherical mirror makes possible not only the processing of a material in the form of a bar having a large diameter but also the reduction in the speed required to rotate the bar.

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[72] Inventors Yoshlhiro Kim; [56] References Cited Yuuhiko Nakayarna, both of Kodoma, UNITED STATES PATENTS Osalmjapan 3 H pp No 857,841 2,354,658 8/1944 219/ 49 UX 22 1 Filed Sept. 15,1969 FORE [45] Patented SepL 7.197] 387,901 l/l924 Germany 219/349 [73] Assignee Mglsmhiu Eledfic Industrial Company, 1 12,869 6/1966 Netherlands 219/349 [1.1- Primary Examiner- R. F. Staubly 08th, J p n Arrorney- McCarthy, Depaoli, O'Brien and Price [54] ARC-IMAGE ZONE-REFINING FURNACE lchim 1 Dawn: ABSTRACTzAn arc-image zone-refining furnace having an [52] 0.8. CI. 219/349, auxiliary spherical mirror located opposite to an ellipsoidal 13/9 mirror. Provision of the auxiliary spherical mirror makes [5 I] Int. Cl. H05b 7/18 possible not only the processing of a material in the form of a [50] Field of Search 219/349, bar having a large diameter but also the reduction in the speed required to rotate the bar.

PATENTED SEP 7 l9?! INVENTOR YOSHIHIRO KINO BY YASUHIKO NAKAYAMA Qu ZLJL M & (963M);

ATTORNEY ARC-IMAGE ZONE-REFINING FURNACE This invention relates to improvement in an arc-image zonerefining furnace and more particularly to a new and improved arc-image furnace provided with an auxiliary spherical mirror whereby a bar of the material to be purified and having a large diameter is effectively processed.

Various techniques of purifying semiconductors, ceramics or the like have heretofore been developed. Among these techniques is an arc-image zone-refining furnace which makes use of a carbon are or xenon arc as the primary source of heat to locally melt these materials. In the furnace of this known type, the radiation emanating from the carbon are or xenon arc is focused into an intense image by one or more ellipsoidal mirrors at the focal point of one of the ellipsoidal mirrors. The

material to be purified, when placed at the focal point of the ellipsoidal mirror, is heated to an extremely high temperature beyond its melting point and melts into liquid in a limited portion thereof. Thus, if the material in the form of a bar is moved relative to the focal point in such a manner that the limited portion, (which will be hereinafter referred to as "liquid zone), traverses the length of the bar, the material of the bar is purified, or more exactly, zone refined and makes a high-purity single crystal bar.

The arc-image furnace as described above, however, has the disadvantage that a bar of the material to be purified is not completely fused in the limited portion of the bar because this particular portion is not heated on its radiation unfocused side because the radiation is intercepted by the bar itself. In order to completely heat this portion, it is the common practice in the art to rotate the bar about its axis at a speed of more than 150 rpm. Rotating the bar, however, places a limitation on the diameter of the bar. As a matter of fact, a bar having a diameter of more than 8 millimeters cannot be processed in this furnace.

It is therefore an object of this invention to provide an improved arc-image furnace using an auxiliary spherical mirror whereby a limited portion of the bar is heated not only on its radiation focused side but also on its radiation unfocused side.

It is another object of this invention to provide an arc-image furnace in which the speed of revolution of the bar is reduced to below 70 rpm. preferably to 40 r.p.m.

It is another object of this invention to provide an arc-image furnace which permits the fabrication of a single crystal bar having a diameter of 12 to 15 millimeters.

The objects and advantages of this invention will be more apparent from the description of embodiment taken in conjunction with the accompanying drawing which shows schematically an arc-image furnace constructed in accordance with one embodiment of this invention.

In the drawing, the arc-image furnace indicated generally at 10 largely comprises, as customary, a radiation source 11 and an ellipsoidal mirror 12. As shown, a radiation source 11 occupies the first focal point of the ellipsoidal mirror 12, which focal point is farther from the ellipsoidal mirror 12. On the contrary a bar 13 of the material to be purified is positioned relative to the ellipsoidal mirror 12 so as to ensure that at least a portion of the bar 13 occupies the second focal point 14 of the ellipsoidal mirror 12, which focal point is closer to the ellipsoidal mirror 12. The radiation emanating from the radiation source 11 is focused into an intense image by the ellipsoidal mirror 12 at the second focal point 14 thereof, so that this particular portion of the bar is heated to extremely high temperatures. Thus, only the portion of the bar 13 is fused to form a liquid zone 15. As is well known in the art, the bar 13 is slowly moved downward relative to the focal point 14, so that a different portion occupies the focal point 14. During the downward movement, the bar 13 is rotated about its axis at a fixed speed by a suitable means (not shown). By so doing, the liquid zone 15 is moved to traverse the length of the bar 13.

According to this invention, an auxiliary spherical mirror 16 is positioned on the major axis of the ellipsoidal mirror 12 and close to the bar 13. The ellipsoidal mirror 12 and the spherical mirror 16 are positioned opposite to each other with respect to the second focal point 14 of the ellipsoidal mirror 12. The spherical mirror 16 focuses at its focal point 17 a major portion of the radiation irregularly reflected from the ellipsoidal mirror 12 and the radiation unfocused on the bar 13. This emanating from the liquid gorge 15 to be regs c heeo for a me, t ereb to lowrt u n I 'd a usment means may be provided 16 so as to keep the ellipsoidal mirror 12 in proper alignment with the spherical mirror 16 so that a desired portion of the bar 13 can be completely fused.

As described above, according to this invention, it is possible to fabricate a high-purity single crystal bar having a large diameter of, for example, l5 millimeters. Furthermore, with the arrangement as proposed, it is not necessary to rapidly rotate a bar of the material to be purified, because the bar is heated on both sides thereof with the ellipsoidal mirror and the spherical mirror.

What is claimed is:

1. An arc-image furnace adapted for zone refining a material, comprising an ellipsoidal reflector, a radiation source positioned at the remote focal point of said ellipsoidal reflector and a relatively small spherical reflector disposed opposite to and facing said ellipsoidal reflector and having its focal point close to the other focal point of said ellipsoidal reflector, the axis of which is disposed on the major axis of said ellipsoidal reflector. 

1. An arc-image furnace adapted for zone refining a material, comprising an ellipsoidal reflector, a radiation source positioned at the remote focal point of said ellipsoidal reflector and a relatively small spherical reflector disposed opposite to and facing said ellipsoidal reflector and having its focal point close to the other focal point of said ellipsoidal reflector, the axis of which is disposed on the major axis of said ellipsoidal reflector. 